Three terminal monolithic multijunction solar cell

ABSTRACT

A device and a method for its fabrication. The device may include a first surface, a second surface to receive light into the device, a first photovoltaic cell between the first surface and the second surface, and a second photovoltaic cell between the first surface and the second surface. The first photovoltaic cell includes a first region of a first photovoltaic material exhibiting an excess of a first type of charge carrier and a second region of the first photovoltaic material exhibiting an excess of a second type of charge carrier, and the second photovoltaic cell includes a first region of a second photovoltaic material exhibiting an excess of the first type of charge carrier and a second region of the second photovoltaic material exhibiting an excess of the second type of charge carrier. 
     A first contact is electrically connected to the second region of the first photovoltaic material, a second contact is electrically connected to the first region of the first photovoltaic material, and a third contact is electrically connected to the first region of the second photovoltaic material. The first surface is between at least a portion of the first contact and the second region of the first photovoltaic material, and the first surface is between at least a portion of the second contact and the second region of the first photovoltaic material.

BACKGROUND

1. Field

Some embodiments generally relate to the conversion of sunlight toelectric current. More specifically, embodiments may relate to improvedphotovoltaic cells for use in conjunction with solar collectors.

2. Brief Description

A solar cell includes photovoltaic material for generating chargecarriers (i.e., holes and electrons) in response to received photons.The photovoltaic material includes a p-n junction which creates anelectric field within the photovoltaic material. The electric fielddirects the generated charge through the photovoltaic material and toelements electrically coupled thereto. Many types of solar cells areknown, which may differ from one another in terms of constituentmaterials, structure and/or fabrication methods. A solar cell may beselected for a particular application based on its efficiency,electrical characteristics, physical characteristics and/or cost.

A multijunction solar cell generally comprises one or more monojunctionsolar cells (i.e., a solar cell as described above) monolithicallyformed on one or more other monojunction solar cells. The photovoltaicmaterial of each of the monojunction solar cells is associated with adifferent bandgap. Consequently, each monojunction solar cell of themultijunction solar cell absorbs (i.e., converts) photons from differentportions of the solar spectrum.

The individual monojunction solar cells of a multijunction solar cellare connected in series. The voltage developed by the multijunctionsolar cell is therefore equal to the sum of the voltages across each ofthe monojunction solar cells. However, the current flowing through themultijunction solar cell is limited to the current produced by itslowest current-producing monojunction solar cell. The excess currentproduced by one or more of the other monojunction solar cells isdissipated as heat, thereby wasting the excess current and elevating thecell temperature. Increased cell temperature typically results indecreased cell efficiency.

Improved monolithic multijunction solar cells are desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The construction and usage of embodiments will become readily apparentfrom consideration of the following specification as illustrated in theaccompanying drawings, in which like reference numerals designate likeparts.

FIG. 1 is a schematic cross section of a device according to someembodiments.

FIG. 2 is a schematic diagram of a system according to some embodiments.

FIG. 3 is a cutaway plan view of a device according to some embodiments.

FIG. 4 is a schematic cross section of a device according to someembodiments.

FIG. 5 is a schematic cross section of a device according to someembodiments.

FIG. 6 is a schematic diagram of a system according to some embodiments.

FIG. 7 is a schematic cross section of a device according to someembodiments.

DETAILED DESCRIPTION

The following description is provided to enable any person in the art tomake and use the described embodiments and sets forth the best modecontemplated by for carrying out some embodiments. Variousmodifications, however, will remain readily apparent to those in theart.

Device 100 of FIG. 1 is a monolithic multijunction photovoltaic cellaccording to some embodiments. Multijunction photovoltaic cell 100includes photovoltaic cell 110 composed of a first photovoltaicmaterial, photovoltaic cell 120 composed of a second photovoltaicmaterial, and photovoltaic cell 130 composed of a third photovoltaicmaterial. Each of cells 110 through 130 includes a region (112, 122 and132) exhibiting an excess of a first type of charge carrier (e.g.,electrons or holes) and a region (114, 124 and 134) exhibiting an excessof a second type of charge carrier (e.g., holes or electrons). Theseregions create respective p-n junctions within each of cells 110 through130, specifically p-n junction 116 within photovoltaic cell 110, p-njunction 126 within photovoltaic cell 120, and p-n junction 136 withinphotovoltaic cell 130.

First surface 140 and second surface 150 are disposed on opposite sidesof device 100. Each of cells 110 through 130 are disposed between firstsurface 140 and second surface 150. Second surface 150 is at leastpartially transparent. In this regard, photons of at least part of thesunlight spectrum may pass through second surface 150 and into device100 during operation of device 100.

Contacts 160, 170 and 190 may be used to extract current from device 100during operation. Each of contacts 160 is electrically connected toregion 114 of cell 110. Each of contacts 170 is electrically connectedto region 112 of cell 110, and electrically insulated from region 114 byvirtue of dielectric insulator 180. At least a portion of each ofcontacts 160 and 170 is disposed on the “back” of device 100. Morespecifically, first surface 140 is between region 114 and at least aportion of each of contacts 160 and 170. Each of contacts 190 iselectrically connected to region 122 of cell 120. Second surface 150 isbetween at least a portion of each of contacts 190 and region 122 ofcell 120.

FIG. 2 is a schematic diagram of system 200 according to someembodiments. System 200 includes a schematic diagram of solar cell 210,which may be implemented by solar cell 100 of FIG. 1. In particular,diode 212 represents photovoltaic cell 120, diode 213 representsphotovoltaic cell 130, and diode 211 represents photovoltaic cell 110.In the illustrated example, and according to conventional multijunctionsolar cell design, a first tunnel diode layer may be disposed betweenphotovoltaic cell 120 and 130, and a second tunnel diode layer may bedisposed between photovoltaic cell 130 and 110. These layers arerepresented by tunnel diodes 220 and 230, respectively. Although notshown in FIG. 1 or FIG. 2, solar cell 100 and solar cell 210 may includeother active, dielectric, metallization and other layers and/orcomponents that are or become known.

Terminals 216, 217 and 219 of solar cell 210 represent contacts 160, 170and 190, respectively. Accordingly, the foregoing arrangement allows theextraction of current generated by photovoltaic cell 110 which exceedsthe current generated by cells 120 and 130. Extraction of this excesscurrent may increase an overall efficiency of device 100 and may loweran operating temperature of device 100 (also resulting in increasedefficiency) with respect to prior arrangements. Embodiments are notlimited to the arrangement of FIGS. 1 and/or 2.

An example of operation will now be provided. Each of the first, secondand third photovoltaic materials is associated with a bandgap. Thebandgap is an energy difference between the top of a material's valenceband and the bottom of the material's conduction band. According to someembodiments, a bandgap associated with the first photovoltaic materialof first photovoltaic cell 110 is less than a bandgap associated withthe third photovoltaic material of third photovoltaic cell 130, and thebandgap associated with the third photovoltaic material of thirdphotovoltaic cell 130 is less than a bandgap associated with the secondphotovoltaic material of second photovoltaic cell 120.

Surface 150 may receive light having any suitable intensity or spectra.Some photons of the received light are absorbed by second photovoltaiccell 120. For example, photons of the received light which exhibitenergies greater than the bandgap associated with the secondphotovoltaic material enter second photovoltaic cell 120 and liberateholes in region 122 and electrons in region 124. The liberated electronsmay be pulled into the region 122 and the liberated holes may be pulledinto region 124 by means of an electric field established by and alongp-n junction 126.

Photons of the received light which exhibit energies less than thebandgap associated with the second photovoltaic material may passthrough photovoltaic cell 120 and into photovoltaic cell 130. Any ofsuch photons which exhibit energies greater than the bandgap associatedwith the third photovoltaic material may liberate holes in region 132and electrons in region 134. Again, the liberated electrons may bepulled into region 132 and the liberated holes may be pulled into region134 by means of an electric field established by and along p-n junction136.

The process may continue within photovoltaic cell 110 with respect tophotons of the received light which exhibit energies less than thebandgaps associated with either the second photovoltaic material or thethird photovoltaic material light 150. These photons which exhibitenergies greater than the bandgap associated with the first photovoltaicmaterial liberate holes in region 112 and electrons in region 114. Theliberated electrons are pulled into region 112 and the liberated holesare pulled into region 114 of photovoltaic cell 110 by means of anelectric field established by and along p-n junction 116.

As described in the present Background, photovoltaic cell 110 maygenerate more current than either of photovoltaic cells 120 or 130.Contact 170 provides an exit path for the excess current so it may beharvested as useful energy. In some embodiments, photovoltaic cell 110is operated as a single-junction solar cell having external contacts 160and 170, while photovoltaic cells 120 and 130 are operated as aseries-connected pair of cells having external contacts 160 and 190.

System 200 illustrates one example of such operation. Inverter 220 iscoupled to terminals 219 and 217 in a typical series-connectedmultijunction cell arrangement. Inverter 230 is coupled to terminals 217and 216 in a typical single junction cell arrangement. In someembodiments, inverter 220 is designed to operate in conjunction with theparticular voltages and currents provided by series-connected cells 212and 213, and inverter 230 is designed to operate in conjunction with theparticular voltages and currents provided by cells 211. Each ofinverters 220 and 230 may be coupled in series or parallel to one ormore other single or multijunction solar cells. The outputs of inverters220 and 230 are connected to provide AC power to an external circuit.

A solar cell according to some embodiments may retain the spectraladvantages of a conventional triple junction solar cell and may befabricated using similar technologies. For example, various layers ofsolar cell 100 may be formed using molecular beam epitaxy and/or metalorganic chemical vapor deposition. According to some embodiments,photovoltaic cell 110 is fabricated according to known techniques andthe remaining photovoltaic cells are fabricated thereon. Each ofphotovoltaic cells 110 through 130 may include several layers of variousphotovoltaic compositions and dopings.

Any suitable materials that are or become known may be incorporated intodevice 100. For example, photovoltaic cell 110 may comprise Ge, GaAs,Si, or any other suitable substrate. Some examples of photovoltaic cell130 include GaAs and GaInP, while examples of photovoltaic cell 120include AlInP, GaInP and AlGaInP.

FIG. 3 is a cutaway plan view of solar cell 300 according to someembodiments. Solar cell 300 may comprise an implementation of solar cell100 and/or solar cell 210 according to some embodiments. The elementsand operation of cell 300 may be similar to those described above withrespect to cell 100.

FIG. 3 illustrates a physical arrangement of contacts 360, contacts 370and dielectric insulator 380 according to some embodiments. Contacts 360are electrically connected to region 312 of photovoltaic cell 310, andcontacts 370 are electrically connected to region 314 of photovoltaiccell 310. The sizes and shapes of contacts 360, contacts 370 anddielectric insulator 380, as well as the relative positions thereof, arenot limited to that shown in FIG. 3. As non-exhaustive examples,contacts 370 and dielectric insulator 380 may exhibit a square or acircular cross section in a plane parallel to second surface 350.

Contacts 390 are electrically coupled to region 322 of photovoltaic cell320. Contacts 390, in some embodiments, are disposed over second surface350 in a grid-like pattern to facilitate suitable collection ofgenerated electrons. Again, any contacts described herein may exhibitany size, pattern or arrangement.

FIG. 4 is a schematic cross section of monolithic multijunction cell 400according to some embodiments. The elements and operation of cell 400may be similar to those described above with respect to cell 100.Moreover, cell 400 may embody cell 210 of FIG. 2.

Contacts 470 of cell 400 are electrically connected to region 412 ofcell 410. However, in contrast to cell 100, contacts 470 extend tophotovoltaic cell 430. Such an arrangement may facilitate fabrication ofcontacts 470 and dielectric insulator 480 in some embodiments. Contacts470 may extend to any suitable degree through region 412 of cell 410.

FIG. 5 depicts a schematic cross section of a monolithic multijunctionphotovoltaic cell according to some embodiments. Multijunctionphotovoltaic cell 500 includes photovoltaic cell 510 composed of a firstphotovoltaic material and photovoltaic cell 520 composed of a secondphotovoltaic material. The first and second photovoltaic materials mayexhibit increasingly larger bandgaps for operation as described above.

Cells 510 and 520 include regions (512 and 522) exhibiting an excess ofa first type of charge carrier (e.g., electrons or holes) and regions(514 and 524) exhibiting an excess of a second type of charge carrier(e.g., holes or electrons). These regions create respective p-n junction516 within photovoltaic cell 510 and p-n junction 526 withinphotovoltaic cell 520.

Cells 510 and 520 are disposed between first surface 540 and secondsurface 550. Second surface 550 is at least partially transparent toaccept light into cell 500 during operation.

Contacts 560 are electrically connected to region 514 of cell 510.Contacts 570 are electrically connected to region 512 of cell 510, andelectrically insulated from region 514 by dielectric insulator 580.First surface 540 is between region 514 and at least a portion of eachof contacts 560 and 570. Contacts 590 are electrically connected toregion 522 of cell 520. Second surface 550 is between at least a portionof each of contacts 590 and region 520.

Cell 500 may be formed using molecular beam epitaxy, metal organicchemical vapor deposition, and/or other suitable techniques. Accordingto some embodiments, photovoltaic cell 510 is initially fabricated andphotovoltaic cell 520 is fabricated thereon. Contacts 560, 570 and 590may be fabricated in any suitable order using any suitable process.

FIG. 6 is a schematic diagram of solar cell 600 according to someembodiments. Photovoltaic cell 500 of FIG. 5 may comprise oneimplementation of solar cell 600. In particular, diode 620 representsphotovoltaic cell 520, and diode 610 represents photovoltaic cell 510.Tunnel diode 615 represents a tunnel diode (unshown) disposed betweenphotovoltaic cells 520 and 510.

Terminals 660, 670 and 690 of solar cell 600 represent contacts 560, 570and 590 of cell 500. Contacts 560, 570 and 590 therefore provide forextraction of current generated by photovoltaic cell 510 which exceedsthe current generated by cell 520, or vice versa. Again, extraction ofthis excess current may increase an overall efficiency of device 500 andmay lower an operating temperature of device 500.

FIG. 7 is a schematic cross section of multijunction solar cell 700according to some embodiments. Solar cell 700 includes photovoltaiccells 710 through 730 composed of respective photovoltaic materials toprovide triple-junction operation as described above.

Similar to the foregoing arrangements, contacts 760 are electricallyconnected to region 714 of cell 710, and contacts 770 are electricallyconnected to region 712 and electrically insulated from region 714 bydielectric insulator 780. First surface 740 is between region 714 and atleast a portion of each of contacts 760 and 770. Each of contacts 790 iselectrically connected to region 722 of cell 720. Accordingly, solarcell 700 may be accurately represented by the schematic diagram of solarcell 210 of FIG. 2.

In contrast to the arrangements described above, first surface 740 isbetween at least a portion of each of contacts 790 and region 712 ofcell 110. That is, at least a portion of each of contacts 760, 770 and790 is disposed on the “back” of cell 700. As a result, front surface750 is not obscured by contacts and is able to receive light over itsentire area. Taken alone, this change may increase an overall efficiencyof cell 700 in comparison to cell 100. However, this increase may beoffset by a decrease in efficiency due to a decreased total volume ofphotovoltaic material. The actual decrease in total volume may becontrolled based on a size, shape and number of contacts 770 and 790.Regardless of the effect on cell efficiency, the presence of allcontacts on the back side of 700 may facilitate electrical connectionthereof to external circuitry.

The several embodiments described herein are solely for the purpose ofillustration. Embodiments may include any currently or hereafter-knownversions of the elements described herein. Therefore, persons skilled inthe art will recognize from this description that other embodiments maybe practiced with various modifications and alterations.

1. A monolithic photovoltaic cell comprising: a first surface; a secondsurface to receive light into the photovoltaic cell; a firstphotovoltaic cell between the first surface and the second surface, thefirst photovoltaic cell comprising a first region of a firstphotovoltaic material exhibiting an excess of a first type of chargecarrier and a second region of the first photovoltaic materialexhibiting an excess of a second type of charge carrier; a secondphotovoltaic cell between the first surface and the second surface, thesecond photovoltaic cell comprising a first region of a secondphotovoltaic material exhibiting an excess of the first type of chargecarrier and a second region of the second photovoltaic materialexhibiting an excess of the second type of charge carrier; a firstcontact electrically connected to the second region of the firstphotovoltaic material; a second contact electrically connected to thefirst region of the first photovoltaic material; and a third contactelectrically connected to the first region of the second photovoltaicmaterial, wherein the first surface is between at least a portion of thefirst contact and the second region of the first photovoltaic material,and wherein the first surface is between at least a portion of thesecond contact and the second region of the first photovoltaic material.2. A cell according to claim 1, wherein the first photovoltaic materialis associated with a first bandgap, wherein the second photovoltaicmaterial is associated with a second bandgap greater than the firstbandgap, and wherein the second region of the second photovoltaicmaterial is between the first region of the second photovoltaic materialand the first region of the first photovoltaic material.
 3. A cellaccording to claim 1, wherein the second surface is between at least aportion of the third contact and the first region of the secondphotovoltaic material.
 4. A cell according to claim 1, wherein the firstsurface is between at least a portion of the third contact and thesecond region of the first photovoltaic material.
 5. A cell according toclaim 1, further comprising: a third photovoltaic cell between the firstphotovoltaic cell and the second photovoltaic cell, the thirdphotovoltaic cell comprising a first region of a third photovoltaicmaterial exhibiting an excess of the first type of charge carrier and asecond region of the third photovoltaic material exhibiting an excess ofthe second type of charge carrier.
 6. A cell according to claim 5,wherein the first photovoltaic material is associated with a firstbandgap, wherein the third photovoltaic material is associated with athird bandgap greater than the first bandgap, wherein the secondphotovoltaic material is associated with a second bandgap greater thanthe third bandgap, wherein the second region of the second photovoltaicmaterial is between the first region of the second photovoltaic materialand the first region of the third photovoltaic material, and wherein thesecond region of the third photovoltaic material is between the firstregion of the third photovoltaic material and the first region of thefirst photovoltaic material.
 7. A cell according to claim 5, wherein thesecond surface is between at least a portion of the third contact andthe first region of the second photovoltaic material.
 8. A cellaccording to claim 5, wherein the first surface is between at least aportion of the third contact and the second region of the firstphotovoltaic material.
 9. A cell according to claim 5, furthercomprising: a first inverter electrically connected to the first contactand to the second contact; and a second inverter electrically connectedto the second contact and to the third contact.
 10. A cell according toclaim 1, further comprising: a first inverter electrically connected tothe first contact and to the second contact; and a second inverterelectrically connected to the second contact and to the third contact.11. A method comprising: fabricating a first photovoltaic cellcomprising a first region of a first photovoltaic material exhibiting anexcess of a first type of charge carrier and a second region of thefirst photovoltaic material exhibiting an excess of a second type ofcharge carrier; fabricating a second photovoltaic cell integral with thefirst photovoltaic cell, the second photovoltaic cell comprising a firstregion of a second photovoltaic material exhibiting an excess of thefirst type of charge carrier and a second region of the secondphotovoltaic material exhibiting an excess of the second type of chargecarrier; fabricating a first contact electrically connected to thesecond region of the first photovoltaic material; fabricating a secondcontact electrically connected to the first region of the firstphotovoltaic material; and fabricating a third contact electricallyconnected to the first region of the second photovoltaic material,wherein the first surface is to receive light into the secondphotovoltaic cell and is between at least a portion of the first contactand the second region of the first photovoltaic material, and whereinthe first surface is between at least a portion of the second contactand the second region of the first photovoltaic material.
 12. A methodaccording to claim 11, wherein the first photovoltaic material isassociated with a first bandgap, wherein the second photovoltaicmaterial is associated with a second bandgap greater than the firstbandgap, and wherein the second region of the second photovoltaicmaterial is between the first region of the second photovoltaic materialand the first region of the first photovoltaic material.
 13. A methodaccording to claim 11, wherein the second surface is between at least aportion of the third contact and the first region of the secondphotovoltaic material.
 14. A method according to claim 11, wherein thefirst surface is between at least a portion of the third contact and thesecond region of the first photovoltaic material.
 15. A method accordingto claim 11, further comprising: fabricating a third photovoltaic cellintegral with the first photovoltaic cell and the second photovoltaiccell, the third photovoltaic cell comprising a first region of a thirdphotovoltaic material exhibiting an excess of the first type of chargecarrier and a second region of the third photovoltaic materialexhibiting an excess of the second type of charge carrier.
 16. A methodaccording to claim 15, wherein the first photovoltaic material isassociated with a first bandgap, wherein the third photovoltaic materialis associated with a third bandgap greater than the first bandgap,wherein the second photovoltaic material is associated with a secondbandgap greater than the third bandgap, wherein the second region of thesecond photovoltaic material is between the first region of the secondphotovoltaic material and the first region of the third photovoltaicmaterial, and wherein the second region of the third photovoltaicmaterial is between the first region of the third photovoltaic materialand the first region of the first photovoltaic material.
 17. A methodaccording to claim 15, wherein the second surface is between at least aportion of the third contact and the first region of the secondphotovoltaic material.
 18. A method according to claim 15, wherein thefirst surface is between at least a portion of the third contact and thesecond region of the first photovoltaic material.
 19. A method accordingto claim 15, further comprising: electrically connecting a firstinverter to the first contact and to the second contact; andelectrically connecting a second inverter to the second contact and tothe third contact.
 20. A method according to claim 11, furthercomprising: electrically connecting a first inverter to the firstcontact and to the second contact; and electrically connecting a secondinverter to the second contact and to the third contact.